Hey there
When making a REST API request to Garmin, Terra will return this message if the request is being made for the very first time for a given period. This is due to the delay in aggregating and forming the data payload on Garmin's end. However, if you re-request for this period, Terra will fetch the data from cache and return it instantly and the same applies for data sent to your webhook, which is already cached.
This only applies to Garmin and is a restriction on their end.